A centrifugal classifier, an inertial classifier, a gravity type classifier and such are used for carrying out a classifying operation for sorting powder into a coarse powder and a fine powder (a minute powder) according to the size of each particle of the powder for the purpose of creating or improving a function required to the powder. Among the above classifiers, the centrifugal classifier is most widely used from the viewpoint of easy control of a particle size, mass processing efficiency, high accuracy in classification and such (See JP-B-S57-24188 and JP-B-S57-24189, for example).
Especially in the various kinds of mining and manufacturing industries, the cement industry, the iron industry and the like, an extremely large quantity of powder subject to classification causes investment in plant and equipment and the running costs (such as expenses for electric energy) to be increased, so that establishment of a technique for decreasing the costs without deteriorating accuracy in classification in a centrifugal classifier is also strongly desired from the viewpoint of not only economical efficiency but also saving in energy of natural resources.
In a centrifugal classifier, a large amount of air or gas is continuously used. Generally, the accuracy in classification is greatly deteriorated when the flow rate of air or gas per unit mass of a powder to be processed is decreased. Such a kind of classifier is also called a centrifugal air classifier.
Further, a fine powder after classification is included in the large amount of air or gas passed through the classifier. In order to collect the fine powder from the air or gas including dust, a large-sized dust collector is required.
Accordingly, establishing a technique capable of reducing the flow rate of the air or gas without deteriorating the accuracy in classification enables a main body of the classifier to be reduced in size, a fan or blower to be reduced in capacity and a dust collector such as a bag filter to be reduced in capacity, so that both costs for plants and equipment and running costs can be reduced.
However, when the flow rate of the air or gas is decreased with a current centrifugal classifier without properly changing a structure of the classifier, the accuracy in classification is greatly deteriorated as described above, and thereby, quality (function) of a powder product and a ratio of collection of a product powder (on any one of the fine powder side and the coarse powder side) are deteriorated. This causes a tendency of deterioration, as a result.